In a nuclear reactor it is important to keep peak centerline temperatures below structural and material integrity limits for safe and controllable operation. Heat energy generated by nuclear reactions is transferred to a coolant and converted into useful forms of energy such as electrical power or propulsion. In plants using liquid coolant, fluid flows through coolant passages and a heat exchange boundary forms where coolant contacts passage surfaces. Coolant passage designs include internal and external flow configurations. In internal configurations, a heated structure at least partially surrounds a perimeter of each coolant passage such that coolant flows in passages within the heated structure. One example of an internal configuration is a block with one or more coolant passages inside the block. The block is cooled with coolant passing through these passages. In an external configuration, coolant flow is external to the heated structure. An example of an external flow configuration is an array of fuel pins as the heated structure, with coolant flowing over the exterior of the pins.
Simple convective heat transfer occurs when coolant is in either a purely liquid or a purely gaseous state. More complex heat transfer occurs during boiling, when liquid coolant transitions to a vapor within a coolant passage. During boiling, heat transfer occurs through three heat transfer mechanisms—heat transfer to liquid coolant, the latent heat of vaporization as liquid coolant transforms to vapor, and heat transfer to coolant vapor. Liquid effectively transfers large amounts of heat, and boiling actually increases heat transfer effectiveness as long a sufficient supply of liquid coolant remains to absorb the latent heat of vaporization. If the liquid coolant boils completely away, however, vapor is all that remains in contact with the passage wall. Vapor is a relatively poor heat transfer medium, and transfers much less heat than liquid coolant. With only vapor left to transfer heat, heat transfer degrades and temperatures can suddenly increase. The point at which the sudden heat transfer degradation occurs is referred to as the Critical Heat Flux (CHF) point, the Departure from Nucleate Boiling (DNB) point, and/or the dryout point. Power generation in the fuel does not halt when the heat transfer degrades, and CHF can result in a temperature excursion within the fuel and clad. These excursions can jeopardize structural or material integrity of the core.
Swirling coolant flow is one way to increase heat transfer and help prevent CHF onset in flowing coolant. Inducing swirling flow can delay and/or prevent the onset of CHF by creating a pressure gradient within a coolant passage. Swirling the coolant creates a pressure gradient towards the center of rotation. For example, in an internal flow configuration, swirling the coolant lowers pressure at the center of a passage relative to the pressure on passage walls. Coolant vapor, being less dense than liquid coolant, is more responsive to the pressure gradient and moves toward the passage center more readily than liquid coolant. This keeps passage walls wetted with liquid coolant rather than coolant vapor, delaying or preventing the onset of CHF. Swirling flow also increases single-phase heat transfer effectiveness. In single-phase heat transfer, swirling flow speeds up coolant velocity over passage walls, increasing heat transfer.
In existing designs, swirling flow is weak due to the use of straight passage walls. FIGS. 1-3 illustrate exemplary flow structures present in triangular, rectangular, and elliptical straight-walled passage cross-sections. Swirling flow velocity in these structures is only approximately 1% of the axial coolant flow velocity. To increase swirling flow (swirl), these designs utilize coolant fins or rifling. Using vanes or rifling to coolant passages is problematic, however, as existing manufacturing constraints restrict physical access along the full length of coolant passages. Moreover, even with fins or rifling, it is difficult to induce swirling flow in non-circular coolant passages. Thus, a need exists for coolant passages having non-circular cross-sections with non-vaned swirl mechanisms.